1. Field of the Invention
This invention relates generally to black and white point detection routines, and more particularly to a system and method in a scanner for automatically detecting the black and white points of a color image.
2. Description of the Background Art
Optical scanners are widely used in image processing. These scanners capture analog pixel signals representing color images from media such as paper or photographs, convert the captured pixel signals to digital image signals, and typically forward the digital image signals to image processors.
The color of a first sheet of "white" paper may differ from that of a second sheet of "white" paper. Although both papers may be initially perceived as white, the papers nevertheless may prove to have different coloring when closely compared with one another. The light color of the paper is referred to as the "white point" of the document. Similarly, perceived "black" text on one medium may in actuality be different from perceived black text on another medium. The dark text color is referred to as the "black point" of the document. To accommodate different background and text colors of color images, modern scanners detect the image black and white points for re-scaling the scanners' profiles, detecting image black or white regions and maintaining image midtones.
For example, when a newspaper article is copied onto white paper, it may be preferred not to copy the newspaper "gray" background color onto the white paper. The modern scanner obtains the white point of the newspaper and of the white paper to eliminate the newspaper gray background color before printing the image on the white paper. By filtering out the background color before making the copy, unnecessary use of toner material may be avoided.
Further, when copying a yellowed original having only black and white information on a color reproduction system having a black toner reproduction unit and a color reproduction unit, the information is reproduced by the black toner unit while the background color must be reproduced by the color reproduction unit. This results in unnecessary and inefficient use of the color reproduction unit.
Therefore, an improved system and method are needed to automatically detect the black and white points of an image, particularly in a document having dark text on a light background.